In general, planar workpieces, such as display panels used in small size display devices, are regularly arranged, e.g. forming a lattice, in one plane and held in a tabular tray for storage or transportation. Those workpieces are picked up one by one sequentially by a transfer mechanism such as a robot from the tray stacked in a loader and supplied to a workstation where components are mounted onto each one of the workpieces. Such a transfer mechanism is disclosed in, e.g. Japanese Patent Application Non-Examined Publication No. H1-115526. Each one of the workpieces undergone the process at the workstation is then held sequentially into an empty tray held by an unloader.
In the foregoing loader and unloader, two tray holders, holding trays in stacked manner, are often placed in parallel. The loader picks up a workpiece from a tray taken out from a first tray holder, and the unloader puts the workpiece in a tray. Those trays are transferred by a tray transfer section, and stacked in a second tray holder.
However, according to the foregoing structure, the workpieces arranged in a two dimensional way should be picked up from the tray, so that the workpiece transfer mechanism needs a mechanism to deal with a pick-up from the two dimensional position. A conventional loader and unloader must incorporate the tray transfer section in addition to the workpiece transfer section. Those necessary elements make it difficult to downsize the loader and the unloader.